The present invention relates to a peak factor reduction device and a base station, and particularly to a peak factor reduction device applied to an RF signal transmitter.
In order to enhance frequency utilization efficiency in a recent wireless communication field, greater importance has been placed on an orthogonal frequency division multiplexing (OFDM) system which modifies each individual base band signal with respective sub-carriers orthogonal to each other and transmits the same. Since the OFDM-modulated signal has a property close to a normal distribution, an instantaneous power ratio relative to average power i.e., a peak factor (peak to average power ratio) (or crest factor) becomes high. If sufficient linearity is not secured for the input/output characteristics of a power amplifier where such a signal is transmitted, then non-linear distortion occurs outside a transmission frequency band. This non-linear distortion becomes an interference signal to other wireless systems, and influences them.
One of measures taken against the interference signal is that the operating point of the power amplifier is lowered to ensure backoff for saturation output power, which improves the non-linear distortion outside the transmission frequency band. In the case of a signal large in peak factor as in the OFDM system, however, larger backoff becomes necessary. Therefore, transmission power efficiency of an RF signal transmitter is reduced. As a result, there is fear that power consumption for a device will increase.
As methods for solving such a problem, there are known: (1) a predistortion or feed-forward distortion compensation technique which compensates for non-linear distortion of the power amplifier and extends the operating range of the amplifier, and (2) a technique which reduces the occurrence of a peak factor at each base band signal. The present invention relates to the latter.
As a document that discloses the latter, may be mentioned, a peak factor reduction device described in a patent document 1 (JP-A-2003-124824). The peak factor reduction device of the patent document 1 includes a reference filter for band-limiting a complex input signal including two types of white base band signals having a uniform spectrum as real and imaginary parts, respectively, a delayer for delaying the complex input signal by a time corresponding to the propagation delay of the reference filter, an amplitude control unit or section for outputting a complex impulse signal having an amplitude proportional to an excess portion when an amplitude component of an output signal of the reference filter exceeds a set value, and a subtractor for subtracting the output signal of the amplitude control unit from the output signal of the delayer.
In the peak factor reduction device described in the patent document 1, a peak factor threshold value Vt is of a fixed value. It is necessary to set Vt in advance. When Vt to be set is low at this time, error vector magnitude (EVM) indicative of an effective value of a vector error increases. Since the increase in EVM influences the quality of a signal at transmission, the allowable value has been defined in terms of the standard. On the other hand, when the set threshold value is set high, EVM is low but the peak factor value becomes high. Hence, a burden on a power amplifier becomes larger. Namely, the setting of EVM and the peak factor threshold value is placed in a trade-off relationship.
Since the allowable value of EVM has been determined in terms of the standard in a large number of wireless base stations, the peak factor threshold value Vt may preferably be set low to such a degree that it does not exceed the EVM allowable value. Depending on transmission conditions such as a transmission frequency bandwidth, a detuning frequency between carriers, the optimum peak factor threshold value Vt changes because of change to frequency of occurrence of the peak. It is also not possible to calculate their values uniformly from those transmission conditions.
When multi-carrier transmission is performed, the phenomenon that the beat goes up occurs as the detuning frequency is detuned. In order to explain the beat phenomenon, an amplitude waveform where a base band signal of a bandwidth of 5 MHz is transmitted two carriers is shown in FIGS. 1A and 1B. FIG. 1A indicates a detuning frequency of 5 MHz, and FIG. 1B indicates a detuning frequency of 15 MHz. The vertical axes of FIGS. 1A and 1B indicate the absolute value of amplitude, and the horizontal axes thereof indicate time. In FIGS. 1A and 1B, fine beats are up as the detuning frequency becomes high.
Next, peak detection where a peak detection width N is wide (N=29) and narrow (N=5) at the beat phenomenon of FIG. 1B will be shown in FIGS. 2A and 2B. FIG. 2A shows where the peak detection width N is wide (N=29), and FIG. 2B shows where the peak detection width N is narrow (N=5). The vertical axes of FIGS. 2A and 2B indicate the absolute value of amplitude, and the horizontal axes thereof indicate time. As is apparent from FIGS. 2A and 2B, the detected number of peaks is low when the peak detection width N is broad. Also the detected number of peaks increases when the peak detection width N is narrow.
When signal levels each equivalent to the same level exist within samples at the peak detection width N where the peak detection width N is wide, the offsetting of peaks becomes incomplete and the peaks remain. On the other hand, when the peak detection width N is narrow, the peaks are excessively detected and EVM after a peak factor reduction becomes degraded. Therefore, it is necessary to set the peak detection width N to a value appropriate according to the transmission conditions. For this problem, the peak factor reduction devices have heretofore been stacked in multistage to prevent the peaks from remaining. However, this configuration will be not preferable from the point of view of implementation.
The present invention aims to provide a peak factor reduction device which makes peak offsetting perfect without increasing the logic scale, and a base station thereof.